WWII Jet Engine Power

I've always wondered why piston-powered jet engines did not see extensive use during ww2?

I know there was a little research and even a few aircraft built but nothing substantial.

Most of the problems of creating a jet engine special materials revolved around the turbine section of the engine. It seems to me that if they had eliminated the turbine section and powered the compressor with a piston engine instead, that a high-thrust engine could have been produced with relative ease. Say, using an R-2800 to power a compressor. Dumping 2000HP into a compressor should be suffice to produce enough thrust to power an aircraft like the ME-262.

I suppose I should put the pencil to paper and do some calculations.... But I wanted to see what everyone here had to say first. I'll post my findings later.
Thanks

Not an expert by any stretch but are you suggesting that an aircraft carry a 2000hp piston compressor to provide high volumes of air into a second engine producing a jet exhaust? Even to me that sounds highy redundant.

This is not as silly as it may sound to some, effectivley what you have is a ducted fan, the piston engine sits where the combustion chamber and turbine would be. Unfortunatley what you also get is a lot of vibration from the piston engine which is what makes it unsuitable.

As for why not just stick a prop on it, you might as well ask why we have turbojets/turbofans when we also have turboprops.

OK, time to define terms. One might as well ask why so many different types of hammers? or scewdrivers, or guns. The proposal was a PISTON powered compressor which in turn supplied air to a jet engine.
A DUCTED fan does not use a piston powered compressor though a piston powered engine could be employed to turn the fan. The fan is in efect a propeler but the duct part reduces losses in thrust from the tip vortices of the fan/propeller, and varying the cross-section of the duct allows the designer to affect the velocity and pressure of the airflow. In addition, ducted fans normally have more and shorter blades than propellers and thus can operate at higher rotational speeds.
The turbojet is the oldest kind of general-purpose airbreathing jet engine, dating back to the late 1930s. The He 178 was the world’s first aircraft to fly purely on turbojet power. Turbojets consist of an air inlet, an air compressor, a combustion chamber, a gas turbine (that drives the air compressor) and a nozzle. The air is compressed into the chamber, heated and expanded by the fuel combustion and then allowed to expand out through the turbine into the nozzle where it is accelerated to high speed to provide propulsion. Turbojets are quite inefficient if flown below about Mach 2 and very noisy. Most modern aircraft use turbofans instead for economic reasons.
The turbofan also breaths air and is widely used for aircraft propulsion. The turbo portion refers to a gas turbine engine which takes mechanical energy from combustion, and the fan, a ducted fan which uses the mechanical energy from the gas turbine to accelerate air rearwards. The engine produces thrust through a combination of air bypassing the core (jet thrust) and air passing through the core (fan thrust); engines that use more jet thrust relative to fan thrust are known as low bypass turbofans, while those that have considerably more fan thrust than jet are known as high bypass. Most commercial aviation jet engines in use today are of the high-bypass type, and most modern military fighter engines are low-bypass.
A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller. The engine's exhaust gases contain little energy compared to a jet engine and play only a minor role in the propulsion of the aircraft. Turboprops are very efficient at flight speeds below 450 mph (725 km/hr) because the jet velocity of the propeller (and exhaust) is relatively low. Due to the high price of turboprop engines, they are mostly used where high-performance short-takeoff and landing capability and efficiency at modest flight speeds are required. In small commuter aircraft, the greater reliability of the turbo (compared to piston) offsets their higher initial cost. Turboprop airliners burn two-thirds of the fuel per passenger as a turbofan. Turbojets, on the other hand, can fly at high altitude for enhanced speed and fuel consumption.

I've got another definition for you Mike, Motojet. A compressor powered by a piston engine, fuel injected into the rear section to provide thrust. The advantage, if there was any, no exotic metals required for the output turbine, because there is no exhaust turbine.

Not just Caproni Campini, tried this, but the Mig I-250, and Sukhol SU-5 tried variations of the motojet.

I agree the main practical advantage of the motorjet in situation of early jet development (and the idea was proposed even long before jet engines existed) is no need to build a turbine with can function in the high temperature exhaust. In fact even today though high temperature metallurgy and turbine blade cooling schemes are highly advanced, an important practical limit on engine performance and efficiency is maximum temperature allowed at the inlet to the HP turbine. With a externally powered compresser, you can avoid that limit (or it's much less restrictive in terms of the temperature a combustion chamber can stand).

However the somewhat practical motorjet arrangements, as mentioned the Sovet VDRK type propulsor used on I-250 and Su-5, also included a propeller for slow speed flight. So, the limitation of propeller drag at very high speed would still exist to a degree.

You are not just 'dragging around' the compressor's piston engine. It's producing useful compression work a turbine driven compressor doesn't have to do, thus energy you don't have to extract from the exhaust stream via the turbine. But AIUI practically speaking a motorjet might produce 50% more or perhaps twice as much total power, from both the compressor engine and the combustion chamber, as the compressor motor would if just attached to a propeller by itself. So again in the very early phase of jets this doesn't look so bad, but the turbojets of definitive postwar fighters produced a lot more power than WWII prop engines; the afterburning generation, albeit bigger a/c produced, on the order of 10 times the power of WWII fighters. This power scale up was much easier to do with the simple mechanism of turbine driven compressor than piston engine driven compressor.

Also, the jet engine naturally transitions from low speed operation where the turbine driven compressor is doing a lot of work relative to the total power output, to high speed operation where the high speed intake air flow is unloading the compressor and turbine relative to the total output. You could do this in theory too with the motorjet, operate it more and more like a ramjet as speed increases but it involves understanding and engineering of fluid dynamics that wasn't as advanced back then.

I think the total engine wieght to power ratio would be much worse than either pure piston prop or just turbojet ratios, that'd be the main snag methinks for military usage - all that extra wieght would require more fuel weight and space to be carried for missions, and so larger A/C with lower cruise speeds per weight to power ratios etc.

Post war Napier did just that and produced tha Nomad; a whopping two stroke 12 cylinder sleeve valve beastie, the compound diesel/gas turbine, which was designed to use a wide range of different fuels. The compressor section was 12 stage and was mechanically coupled to the piston engine. It put out 3,037 shp. It was intended to be fitted to Shackletons, but the idea was killed off by the turbo prop.

One thing that is worth mentioning, is that in the big high bypass turbo fan engines, like the RR Trent, 90 percent of the total thrust is produced by the fan at the front; the turbine is there to drive the compressor stages, although that 10 percent of thrust in some of those big fans is still equal to about 10,000 lbs, it is only a small portion of total. On almost all your commonly used turbo props, thrust is produced by the propeller; the hot stuff that comes out the back is just that. Only on big turbo props like the Russian ones and the Allison T-56 (C-130, P-3) does the hot stuff produce a bit of thrust.

Post war Napier did just that and produced tha Nomad; a whopping two stroke 12 cylinder sleeve valve beastie, the compound diesel/gas turbine, which was designed to use a wide range of different fuels. The compressor section was 12 stage and was mechanically coupled to the piston engine. It put out 3,037 shp. It was intended to be fitted to Shackletons, but the idea was killed off by the turbo prop.

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Slightly different kettle of fish nuumannn.

The Nomad was a compound engine. Even the first version with on prop driven driectly from the turbine had the other driven by the Diesel engine. The concept thet is being discussed here is to dispense with the turbine and use a piston engine to compress air for a combustor and exhaust nozzle to create thrust.

use a piston engine to compress air for a combustor and exhaust nozzle to create thrust.

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Okay, I know, but the compressor was mechanically coupled to the piston engine, but I see your point. I mentioned the Napier because a place I used to work at had one and it always impressed me; it's such a big and complex beastie - the guys working on it only got it fitted out, but not to a working state and it would have been so impressive seeing that thing roaring away on a test stand.

I guess the concept of a reciprocating engine driving a compressor didn't get to be put into practice because prior to the war there were a few people working on pure gas turbine engines, which offered a significant advantage over such a thing. Why go to all that effort when a centrifugal flow gas turbine has fewer moving parts than a V-12 piston engine and can produce enough thrust to push along an aircraft of similar size to your average fighter at a higher speed? Also, the first axial flow gas turbine was bench tested before the war, so development of jet engines would be more advantageous if you are going to go to the effort to research compressor design.

Which one ???

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Yep, the Baka was powered by a rocket engine; one variant was powered by a gas turbine.

Okay, I know, but the compressor was mechanically coupled to the piston engine, but I see your point. I mentioned the Napier because a place I used to work at had one and it always impressed me; it's such a big and complex beastie - the guys working on it only got it fitted out, but not to a working state and it would have been so impressive seeing that thing roaring away on a test stand.

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The compressor fed the piston engine, and nothing else as far as I know. There was a facility for burning extra fuel in the exhaust, due to there being excess oxygen available.

I guess the concept of a reciprocating engine driving a compressor didn't get to be put into practice because prior to the war there were a few people working on pure gas turbine engines, which offered a significant advantage over such a thing. Why go to all that effort when a centrifugal flow gas turbine has fewer moving parts than a V-12 piston engine and can produce enough thrust to push along an aircraft of similar size to your average fighter at a higher speed? Also, the first axial flow gas turbine was bench tested before the war, so development of jet engines would be more advantageous if you are going to go to the effort to research compressor design.

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One of the big advantages of gas turbines over such arrangements was weight - they weighed comparatively little.

As stated before, getting the alloys to work for the turbines was an issue. The British and Americans had decent alloys that could cope, but the Germans didn't have enough of the alloying components, so were developing air-cooled blades by the end of the war.

Such a "concept of a reciprocating engine driving a compressor" was put into practice.

Say, using an R-2800 to power a compressor. Dumping 2000HP into a compressor should be suffice to produce enough thrust to power an aircraft like the ME-262.

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I wouldn't be too keen on using an R-2800 - simply because of its size. Also, a liquid cooled engine would allow more freedom in locating components. The Caproni-Campini N1 seems to have a long path between the compressor and the combustor.

While the motorjet solved one problem (turbine) design, it combined several of the worst problems of both the jet and piston engine.

The ducted version like the Caproni Campini N.1 is going to have higher drag than a conventional prop airplane. Not only from the larger, fatter fuselage but since drag increases with the square of the speed, trying to move air any long distance through the fuselage is going to create drag, most ducted fans keep the duct as short as possible. While the theory may look good, in practice whatever benefit is gained in the propulsion set up has to balance against the higher weight and drag of an airplane with this type of engine compared to a plane of the same speed/payload using a conventional engine propeller.

The fuel economy is going to be atrocious. Not only are you running the piston engine but the combustion chamber "jet" reaction set up is far from efficient. Just a like a piston engine, the economy of a JET engine depends a lot on the compression ratio. Early jet engines ran on a compression ratio of about 3 or 4 to 1. The lower you go the less LESS thrust you get per pound of fuel burned. It is possible to get thrust and speed but the endurance is going to be very short and your total airplane weight has to contend withe weight of the piston engine to boot.

Let us assume for arguments sake, that the R-2800 could power one or more compressors through shafts that could generate 3600-3800lbs of thrust. Say we stick this "engine" in an Arado 234. R-2800 in the fuselage, gear box and shafting go to a compressor in a duct under each wing where the regular jet engines were. Each Jumo 004 weighed 1,585 lb ? how much was the turbine section (1 stage) and how much was the burner section and how much was the compressor section (8 stages)? Now how much are the gear boxes and shafting going to weigh to get the power to the motojets? The R-2800 goes about 2400-2600lbs (dry) and can suck fuel at 1500lbs an hour at max continuous rating. A Jumo 004 used 1.4lbs fuel per 1lb of thrust per hour, or 5320lbs per hour for a pair. Even if you adjust that down ward to take into account that the jets no longer have to turn their own compressors that is a lot of fuel. But the R-2800 (installed) and its 1 hour of fuel are worth around 1 hours worth of fuel to the Arado 234 depending on the installed weight.

Getting an idea to work is one thing, getting it to be PRACTICAL is another.

The Russian Mig and Sukhol developement of the motojet used the piston engine as the primany power and the motojet as a booster at the rear. Evidently a coupling of some sort could be engaged to power the motojet, boosted top speed into the high 400 to low 500mph range, like a 40-50 mph boost.

It must not of been the success Stalin was wanting, several Soviet engineers went the gulag when the projects were cancelled. Talk about a high pressure work enviroment !